We have been studying the synthesis and processing of the two cytoplasmic precursors (C' and D') of the nuclear low molecular weight RNA species C and D. These RNAs are transcribed in the nucleus, their precursors are quickly found in the cytoplasm, (apparently as an in vivo phenomenon) where they remain for a few minutes, and finally they return to the nucleoplasm, where they are long lived. Our studies have included fingerprinting, kinetics of their appearance and disappearance from subcellular fractions, non-aqueous cell fractionation, kinetics of nucleotide loss, formation of methylated 5' terminal caps, and particle association in the cytoplasm. Our current goal is the study of the synthesis and processing of RNAs C and D, as well as other low molecular weight RNAs in mammalian cells. This includes continuation of our work on in vivo methylation of C' and D', and their apparent particle association, plus studies on in vivo pseudouridine formation during maturation of small RNA precursors, and synthesis of low molecular weight RNAs in cell-free systems. We will also study the in vivo effect of several treatments that differentially affect the synthesis of various types of RNAs in mammalian cells: by acting on DNA (ultraviolet irradiation, actinomycin D, capmtothecin), by affecting an RNA polymerase (a-amanitin), by depletion of a precursor pool (galactosamine, glucosamine), as a substrate analogue (toyocamycin, 3' deoxycytidine, formycin, cordycepin) or by mechaisms less well understood (high temperature, hypertonicity of the culture medium).